Turkish President Recep Tayyip Erdogan has reaffirmed his country's interest in purchasing a missile defense system from Russia despite concern about the deal among Ankara's NATO allies.

"We attach great importance to the joint steps Turkey and Russia will take on defense," Erdogan said after several hours of talks with Russian President Vladimir Putin at a meeting in Sochi on November 13.

Turkey recently agreed to purchase the Russian S-400 surface-to-air missile systems in a move seen as a snub by some NATO allies, which pointed out that the weapon cannot be integrated into the alliance's defenses.

Although Turkey confirmed that it was going ahead with the missile deal, worth around $2 billion, it is not expected to take delivery of the systems for another two years.

The purchase from Russia opens Ankara up to the possibility of coming under U.S. sanctions recently announced against Russia's defense industry. But Pentagon officials have said they are in talks with Ankara to try to avoid that outcome.

Syrian Peace Efforts

Erdogan and Putin said they had a "productive" meeting discussing efforts to end the conflict in Syria, where the two nations are on opposing sides but have been co-sponsoring peace talks in Kazakhstan.

"Our joint work with Turkey and Iran has brought concrete results," Putin said. "Violence has abated, and favorable conditions have been created for advancing an inter-Syrian dialogue under the UN auspices."

Putin reportedly briefed Erdogan on an agreement he reached with U.S. Donald Trump on Syria in Asia over the weekend that said both countries were committed to reviving long-stalled UN peace negotiations.

After meeting with Erdogan, Putin said: "We are united in the need to increase efforts to ensure the long-term stabilization [of Syria], above all to advance the process of a political settlement."

"We agreed that there is now a base which allows us to focus on the political process," Erdogan said.

Separately, U.S. Defense Secretary Jim Mattis said the United States would forge ahead with its fight against the Islamic State extremist group in Syria until a UN peace process makes further headway.

"We're not just going to walk away right now until the Geneva Process has traction," he told reporters in Washington.

"You need to do something about this mess now, not just fight the military part of it and then say good luck on the rest."

"We are going to make sure we set the conditions for a diplomatic solution," Mattis said.